Rectifier Side Research Articles

Reactive Power Control Method for the LCC Rectifier Side of a Hybrid HVDC System Exploiting DC Voltage Adjustment and Switched Shunt Device Control

We present a new reactive power control method for hybrid high-voltage direct-current (HVDC) systems that features a line-commutated converter-based (LCC) rectifier and a voltage-sourced converter-based (VSC) inverter. Our principal objective is to regulate the reactive power of the LCC rectifier side via co-operative control of the DC voltage and switched shunt devices (SSDs) such as capacitors and reactors. To achieve these objectives, we used three operative modes: reactive power, voltage, and power factor control. For each mode, we show how to determine the DC reference voltage. Also, to maintain the voltage within operational bounds and enhance the range over which reactive power is controllable, our method features the cooperation of SSDs. Furthermore, we show how to determine the required SSD capacities. MATLAB and PSCAD simulations confirmed that our method improves reactive power controllability at the LCC rectifier side of a hybrid HVDC system.

A Coordinating Control for Hybrid HVdc Systems in Weak Grid

A coordinating control strategy is proposed for a hybrid high voltage dc (HVdc) system comprising a capacitor commutated converter connected in series with a two-stage voltage source converter called “Vernier.” This approach ensures that 1) the tap changers and switched capacitor banks are eliminated by Vernier controls that maintain constant firing angle and margin angle at the rectifier and inverter, respectively, and provide volt-VAr control at the commutation bus, and 2) improves the power flow recovery and risk of commutation failure following sever faults. To that end, a detailed switched model of hybrid-HVdc is built in EMTDC/PSCAD platform. For the frequency-domain analysis, a new nonlinear state-space averaged model is also developed and benchmarked against the detailed model. The steady-state operating characteristics of the proposed hybrid HVdc system are established. Finally, the effectiveness of the proposed approach is demonstrated for weak ac systems interfacing the rectifier and inverter sides. Improvement in commutation failure and power flow recovery following severe disturbances are also shown through case studies.

A Mathematical Framework for Modelling of Current Source Converter Based High Voltage DC Transmission Systems

Transmission of electrical power through High Voltage Direct Current (HVDC) has attracted the attention of several researchers in the recent years. To investigate the performance of HVDC transmission systems, a complete linear mathematical model is required. In this paper, a well-developed linear continuous model of current source converter based twelve pulse HVDC transmission systems is presented. In which, the converter AC system is represented by damped LLR equivalents at fundamental frequency and at the third harmonic. Also, they are equipped with double tuned harmonic filter and second order high pass filter to suppress the AC harmonics and a capacitor for reactive power compensation. The DC system is secured with rectifier current control, inverter current control, inverter voltage control and inverter extinction angle control. The HVDC transmission system model is implemented in the MATLAB/Simulink environment and the performance of the system has been investigated by observing the rectifier side AC quantities, rectifier DC quantities, inverter side AC quantities and inverter DC quantities.

Pearson correlation coefficient of current derivatives based pilot protection scheme for long-distance LCC-HVDC transmission lines

This paper proposes a pilot protection scheme for long-distance LCC-HVDC transmission lines based on Pearson correlation coefficient of current derivatives. According to the impedance frequency characteristics of converter and DC filter, and the superposition principle, the correlation characteristics between line current derivative (LCD) and DC filter current derivative (DFCD) are analysed for internal and external faults. For internal fault, there is completely negative correlation between line current derivative and corresponding DC filter current derivative at both rectifier side and inverter side; for external fault, the correlation of current derivatives (CDs) at the faulty side is positive, yet completely contrary at the other side. The Pearson correlation coefficient (PCC) is utilized to detect the correlation characteristics of current derivatives. And based on these characteristics, a novel protection scheme for long-distance HVDC transmission lines can be constructed to detect internal and external faults. Comprehensive simulation results show that the proposed protection scheme can quickly identify internal and external faults with high reliability. Furthermore, it is insensitive to fault resistance, fault location and measurement noise, and does not require data synchronization.

A Novel Hybrid Control Method for Single-Phase-Input Variable Frequency Speed Control System With a Small DC-Link Capacitor

A novel hybrid control method, which consists of dual-frequency tracking control (DFTC) and virtual inertia control (VIC), is proposed for single-phase-input variable frequency speed regulation system with a small dc-link capacitor. Because of lacking a large-capacity energy storage device, the dc-link voltage is a pulsating dc, which causes a strong power coupling relationship between the rectifier and inverter side. The instantaneous power and dq -axis currents of the inverter contain both ac and dc components at the same time, which cannot be controlled well using traditional proportional-integrational (PI) controllers. Herein the DFTC that is based on a PI-resonant controller is proposed to accurately track ac and dc components of the instantaneous power and dq -axis currents respectively. With higher open-loop gain at both low frequency and the specified resonant frequency, the proposed DFTC ensures accurate tracking of the ac and dc components of power and dq -axis currents, high-power-factor, and low total harmonic distortion (THD) of input grid current. To maintain the stability of the dc-link voltage under step changes of load, a VIC is proposed. Subsequently, the input power factor and the fluctuation of dc-link voltage can be improved in the system. Simulations and experimental results are given to provide further validation of the proposed DFTC and VIC under different operating scenarios.

DC-side harmonic analysis and DC filter design in hybrid HVDC transmission systems

Hybrid high-voltage direct current (HVDC) transmission is a new transmission structure that uses line commutated converter (LCC) at the rectifier side and voltage source converter (VSC) at the inverter side. This study proposes a method to calculate DC side harmonic currents of hybrid HVDC transmission systems and design filters to mitigate DC-side low-frequency voltage pulsations. First, an equivalent harmonic analysis model of modular multilevel converter (MMC) is deduced according to the relationship among different harmonic components. The harmonic voltage expressions at DC side of 12-pulse LCC are deduced by combining the three-pulse harmonic voltage source model and FFT decomposition. On this basis, the expressions for harmonic currents are derived by multiplying the harmonic voltage transfer function and transfer admittance equation of DC network and are solved for the inverse Laplace transform. Two different types of DC filters are designed according to the requirements of DC filter on decaying specific order harmonic currents. The influence of partial element decay on the transfer characteristic of filter is analyzed, and appropriate partial component dissipation parameters are applied. The decay pole near the harmonic resonance frequency is configured to achieve reasonable DC-filtering results. Simulation results of a hybrid HVDC transmission system demonstrate the effectiveness of the proposed analysis model and filter design method.

Zero-Voltage-Switching SiC-MOSFET Three-Phase Four-Wire Back-to-Back Converter

This paper proposes a novel zero-voltage-switching (ZVS) three-phase four-wire back-to-back (BTB) converter topology and its ZVS sinusoidal pulsewidth modulation (ZVS-SPWM) scheme. With the proposed ZVS-SPWM scheme, all the main switches in both the rectifier side and inverter side and the auxiliary switch can realize ZVS turn on with only one auxiliary circuit. In addition, the auxiliary switch only needs to operate once in each switching period, which can cut down an extra loss of the auxiliary circuit. The operation stage analysis is introduced, and the ZVS condition for the switches is derived. The ZVS BTB converter is investigated with respect to both balanced and unbalanced load conditions. In additon, the soft-switching parameter design guideline is discussed. Finally, the proposed ZVS BTB converter and its ZVS-SPWM scheme are verified on a 9-kW prototype with the SiC-MOSFET device.

Optimal Coordination Voltage Control of Hybrid AC/DC Grids Considering Control of HVDC System

With the wide application of UHVDC in long-distance and large-capacity power transmission, the current power grid has developed into a complex AC-DC deep coupling system, which poses a serious challenge to the voltage safety control of the system. Based on the voltage sensitivity of AC/DC system, this paper presents a voltage optimization control method for integrated coordination of AC/DC system control variables. In this control method, a linear constrained quadratic optimization model with minimum control cost is constructed to coordinate the control variables of AC/DC system and correct the voltage amplitude deviation. The potential of UHVDC system to participate in voltage control is studied by optimizing the control quantity of rectifier side and inverter side converters. The simulation results of AC/DC hybrid system show that the proposed optimal control method can effectively coordinate the control quantity of AC/DC system and ensure the voltage operation in a safe range.

A Unidirectional Single-Phase AC–DC–AC Three-Level Three-Leg Converter

A unidirectional ac–dc–ac three-level three-leg converter is proposed in this paper. This converter is composed of a unidirectional neutral-point-clamped (NPC) leg and two bidirectional NPC legs. A bidirectional NPC leg is shared between rectifier and inverter sides. Compared with the two-level leg, the three-level leg presents voltage stress reduction on switches for the same dc-link voltage level and also allows a reduction in harmonic distortion for the same switching frequency. Suitable modeling, pulsewidth modulation, and control strategies of the system are presented. Also, a method for balancing dc-link capacitor voltages is shown. As the converter is unidirectional, the grid current and the generated voltage at the load side must be synchronized with the generated voltage at the grid side, which eliminates the zero-crossover distortion in the grid current caused by the use of diodes without the hysteresis control. Simulation and experimental results are also presented.

Control strategy for hybrid LCC‐C‐MMC HVDC system under AC fault at rectifier side

This paper focus on the new hybrid HVDC system, whose rectifier side is traditional line commutated converter (LCC) and inverter side is made up of the series connected combined modular multilevel converter with clamp double sub-modules (C-MMCs). However, when the AC grid fault occurs at rectifier side, the DC voltage of LCC will drop; at this time, the DC voltage of C-MMC can not be decreased much since the modulation range is limited. Thus, the transmitted power may interrupt if the rectifier DC voltage is lower than the inverter side. To solve this problem, the paper proposed a new low DC voltage ride through control strategy. When the fault is very serious, which means the DC voltage will drop much, the converter unit at high voltage terminal is switched off to make the system operate under the half-voltage mode, then, the healthy converter unit is controlled to transmit power continuously. During the process, one arm is bypassed, and the other two arms are blocked. Finally, the PSCAD/EMTDC results show, the proposed control strategies achieve excellent operation characteristic under 10% and 50% ac voltage drop.

Reliability model and algorithm research on HVDC system and flexible HVDC system

The main difference between flexible high-voltage direct current (HVDC) system and the traditional HVDC system is that there are two control degrees of freedom in the flexible HVDC system that is the amplitude and phase angle of the output voltage. While, there is only one control degrees of freedom in the traditional HVDC system which is the amplitude of the output voltage. Therefore, compared with the traditional HVDC system, there will not be the problem of commutation failure in the flexible HVDC system, and the control systems of the rectifier side and the inverter side being operated independently make the reliability of the system greatly improved. This article analyses and compares the main methods of reliability assessment. Analysing and comparing the reliability parameters of the two systems by using the state enumeration method, and using the economic characterisation to make the comparison more intuitive.

Simulation and analysis of ± 1100 kV UHVDC transmission system with hierarchical connection mode based on PSCAD/EMTDC

This study first introduces the structure and principle of the control system of ultra-high-voltage direct current (UHVDC) transmission system with hierarchical connection mode, mainly including the whole control logic, the voltage balance control module of high-end and low-end converter stations and the commutation failure prediction function. On this basis, this study sets up an electromagnetic transient equivalent model of UHV AC/DC interconnected system with hierarchical connection mode by simulation software electromagnetic transient design and control/power systems computer-aided design (EMTDC/PSCAD) based on Zhundong–Huadong ± 1100 kV UHVDC transmission project. The simulation analysis was first carried out in steady-state operation. Comparing the electrical parameters of the model under the condition of bipolar total voltage with the actual project, the model is consistent with actual engineering parameters. Next, the simulation was undertaken when single-phase grounding fault occurs on AC bus of rectifier side and inverter side. Finally, simulation waveforms and corresponding conclusion are summarised in this study. The results indicate that the DC control system established with hierarchical connection mode under the PSCAD/EMTDC environment has good adjustment effect and can provide a good reference for practise.

Simultaneous DC Current Balance and Common-Mode Voltage Control With Multilevel Current Source Inverters

The current source converter (CSC) has been popular in high-power medium-voltage applications. To increase the system power capacity and reduce the output harmonics, parallel connection of CSCs with multilevel current output is a practical approach. There are mainly two types of parallel based configurations. The structure with an independent dc link for each paralleled CSC can be regarded as the first configuration, where the dc current flowing into each CSC can be controlled from the rectifier side independently. The second type is constructed with only one current source rectifier (CSR) on the grid side and paralleled current source inverters (CSIs) on the load side, which could potentially reduce the cost and size of the system. However, with a common dc source, the dc current for each CSC should be carefully controlled with proper modulation design. At the same time, the common-mode voltage (CMV), which increases the motor line-to-ground voltage and causes additional stress to the insulation system, should also be suppressed. In this paper, with a focus on the shared dc-link structure, we thoroughly analyze the dc-link current flow and common-mode loop circuit. We propose a multilevel modulation strategy of the parallel CSI system considering the requirement of dc current balancing and CMV reduction to improve the overall system performance. The effectiveness of the proposed method is verified by both simulation and experiment in a transformerless parallel CSC system.

Analysis of a High-Power, Resonant DC–DC Converter for DC Wind Turbines

This paper is introducing a new method of operation for a series resonant converter, with intended application in megawatt high-voltage dc wind turbines. Compared to a frequency controlled series resonant converter operated in subresonant mode, the method (entitled pulse removal technique) allows the design of the medium frequency transformer for highest switching frequency, while being operated at lower frequency without saturation. The main focus of this paper is to identify and analyze the operating modes of the converter with pulse removal technique. With the use of variable frequency and variable phase displacement in subresonant mode, the new method of operation promises transformer size reduction and facilitates soft-switching transition of the insulated gate bipolar transistors (IGBTs) and line frequency diodes on rectifier side. Four modes of operation are identified, while equations for output power, voltage, and current stress are identified. Experimental results are concluded on a 1 kW, 250 V/500 V prototype.

A High-Power, Medium-Voltage, Series-Resonant Converter for DC Wind Turbines

A new modulation scheme is introduced for a single-phase series-resonant converter, which permits continuous regulation of power from nominal level to zero, in presence of variable input and output dc voltage levels. Rearranging the circuit to locate the resonant LC tank on the rectifier side of the high turns-ratio transformer combined with frequency control and phase-shifted inverter modulation keep transformer flux constant from nominal frequency down to dc, always in subresonant continuous or discontinuous conduction mode. This overcomes the principal deficit of series-resonant converters, and the resulting compact and efficient transformer, and soft-commutated inverter, present particular advantages in high-power, high-voltage applications, such as dc offshore wind turbines. With transformer excitation frequency in hundreds of Hz range, line-frequency diodes can be employed in the high-voltage rectifier valve. Circuit operation and conduction modes, governing equations, and sample waveforms are presented, together with experiments from a scaled demonstrator.

New Insights Into Coupled Frequency Dynamics of AC Grids in Rectifier and Inverter Sides of LCC-HVDC Interfacing DFIG-Based Wind Farms

Coupling between frequency dynamics of the ac systems on both inverter and rectifier sides of the line-commutated converter HVDC with the rectifier station operating in frequency control is studied, along with the presence of large DFIG-based wind farms on the weak rectifier-side grid. An averaged model with 79 states, which includes dynamic models of grids on the rectifier and inverter sides, phase-locked loop, and the wind farm is derived. To develop a deeper understanding of the frequency dynamics, a simplified four-state nonlinear model is proposed, which, in turn, reveals strong coupling between frequency and ac voltage at the HVDC rectifier terminal. A firing angle correction strategy is proposed to decouple frequency–voltage interactions, thereby improving the frequency dynamics on the rectifier side. The four-state model is linearized to ascertain the interaction between rectifier- and inverter-side frequencies, and an analytical expression for the frequency dynamics in terms of gains of the frequency controller at the rectifier station is derived. Moreover, the proposed reduced-order model shows the implications of frequency droop control of the wind farms in improving frequency dynamics on both rectifier and inverter sides. Expressions for “synchronizing” and “damping torque” contribution from HVDC and wind farm are also established. The analytical expressions and the effectiveness of the proposed strategies are validated through nonlinear time-domain simulations.

Modular Transformer-Based Regenerative-Cascaded Multicell Converter for Drives With Multilevel Voltage Operation at Both Input and Output Sides

Cascaded multicell converter (CMC) is proved to be an effective solution for medium voltage drives. Three-phase diode bridge rectifiers are normally used in this converter, which do not have regeneration capability. Pulsewidth-modulated (PWM) rectifier-based power cells enable the drives with regeneration capability. However, due to their two-level voltage operation at the input side, an inductor is required for each cell to filter out the switching frequency components from the input current. In this paper, a new configuration with single-phase PWM rectifier-based power cell is proposed to achieve multilevel voltage operation at both the input and output sides of the converter. Therefore, the devices at the rectifier side are also operated at reduced switching frequency without using any filter inductor for each cell. In the proposed converter, simple modular transformers are used instead of a single large and complex transformer that is used in the conventional CMC. Further, input phase currents of the converter are controlled in the synchronously rotating d-q frame, which requires only two current sensors at the input side instead of individual current sensors for each cell. Experimental validation of the proposed converter configuration is carried out on a 4.5-kVA prototype, and the results are presented.

Hybrid HVDC (H2VDC) System Using Current and Voltage Source Converters

This paper presents an analysis of a new high voltage DC (HVDC) transmission system, which is based on current and voltage source converters (CSC and VSC) in the same circuit. This proposed topology is composed of one CSC (rectifier) and one or more VSCs (inverters) connected through an overhead transmission line in a multiterminal configuration. The main purpose of this Hybrid HVDC (H2VDC), as it was designed, is putting together the best benefits of both types of converters in the same circuit: no commutation failure and system’s black start capability in the VSC side, high power converter capability and low cost at the rectifier side, etc. A monopole of the H2VDC system with one CSC and two VSCs—here, the VSC is the Modular Multilevel Converter (MMC) considered with full-bridge submodules—in multiterminal configuration is studied. The study includes theoretical analyses, development of the CSC and VSCs control philosophies and simulations. The H2VDC system’s behavior is analyzed by computational simulations considering steady-state operation and short-circuit conditions at the AC and DC side. The obtained results and conclusions show a promising system for very high-power multiterminal HVDC transmission.

A Novel Three-Level Voltage Source Converter for AC–DC–AC Conversion

This paper presents a novel three-level voltage source converter for AC–DC–AC conversion. The proposed converter based on H-bridge structure is studied in detail. The control method with traditional double-closed-loop control strategy and voltage balancing algorithm is applied to the rectifier side. Correspondingly, a simplified modulation algorithm is applied to the inverter side, and the voltage balancing of inverter side is realized through the optimal selection of switching combination. Then, the application of the proposed topology is assessed in general and ideal operation conditions. Furthermore, the proposed topology with a variable voltage variable frequency (VVVF) is verified in experimental conditions. The performance of the proposed converter and control strategy is evaluated by experimental and simulation results.

Design and implementation of perturbation observer‐based robust passivity‐based control for VSC‐MTDC systems considering offshore wind power integration

Voltage source converter-based multi-terminal high-voltage direct current (VSC-MTDC) systems are starting to be commissioned. However, concentrated integration of large-scale wind power demands stronger robustness against power fluctuation and system disturbances to increase the reliability of the whole system. This study proposes a perturbation observer-based robust passivity-based control (PORPC) for VSC-MTDC systems connected to an offshore wind farm to meet the demands. The aggregated effect of system nonlinearities, parameter uncertainties, unmodelled dynamics and external disturbances includes grid faults and time-varying wind power output is estimated by a linear perturbation observer and fully compensated by a passive controller, thus no accurate VSC-MTDC system model is required. PORPC attempts to regulate DC voltage and reactive power at the rectifier side, as well as active power and reactive power at the inverters side connected to an offshore wind farm. Besides, a DC-link voltage droop controller is introduced so as to provide immediate response to the grid unbalance situation. Moreover, a noticeable robustness against parameter uncertainties can be achieved as no accurate system model is needed. Case studies are carried out to compare the performance of PORPC to other typical approaches. Finally, a hardware-in-the-loop test is undertaken via dSPACE which validate its implementation feasibility.